Iron-phosphate coating for tin-plated ferrous metal surfaces

ABSTRACT

A composition and method for treating tin-plated ferrous metal surfaces. The coating produced on the metal surface inhibits corrosion and acts as a bonding agent for subsequently applied organic finishing coats of sanitary lacquer or the like. More specifically, this invention relates to a composition for treating drawn and ironed tin-plated ferrous metal containers. The composition includes an aqueous solution of monobasic or dibasic monophosphates and mixtures thereof, alkali or ammonium fluoride or bifluoride and a hydroxylamine.

United States Patent Jones Oct. 1, 1974 [75] Inventor: Thomas C. Jones,Philadelphia, Pa.

[73] Assignee: Amchem Products, Inc., Ambler, Pa.

[22] Filed: July 27, 1972 [21] Appl. No.: 275,620

[52] US. Cl. l48/6.15 R, 106/14 [51] Int. Cl. C23f 7/12 [58] Field ofSearch 148/615 R, 6.16; 106/14 [56] References Cited UNITED STATESPATENTS 2,160,061. 5/1939 2,298,280 l/1942 2,327,885 8/1943 2,665,231l/l954 2,702,768 2/1955 Hyams et al. 148/615 R Primary Examiner-Ralph S.Kendall Attorney, Agent, or FirmErnest G. Szoke; Howard S. Katzoff 5 7ABSTRACT A composition and method for treating tin-plated ferrous metalsurfaces. The coating produced on the metal surface inhibits corrosionand acts as a bonding agent for subsequently applied organic finishingcoats of sanitary lacquer or the like. More specifically, this inventionrelates to a composition for treating drawn and ironed tin-platedferrous metal containers. The composition includes an aqueous solutionof monobasic or dibasic monophosphates and mixtures thereof, alkali orammonium fluoride or bifluoride and a hydroxylamine.

2 Claims, No Drawings IRON-PHOSPHATE COATING FOR TIN-PLATED FERROUSMETAL SURFACES BACKGROUND OF THE INVENTION Containers used in thefoodstuff industry can be made of tin-plated ferrous metal sheeting.These containers are ordinarily formed through a process referred to asDrawing and Ironing, that is the tinplated metal sheeting is drawn andintentionally thinned to form a drawn sheet providing uniform wallthickness and producing a thin walled and thick bottomed container.Because tin plate is expensive, only a thin tin-plating is employed. Aserious problem with drawn and ironed tin-plated containers is that thedrawing operation stretches the interior tin-plate surface therebyproducing pores which leave the underlying steel base exposed. Thisallows the contents in the container to react with the underlying steelbase surface thereby providing contaminants which give the contents anunpleasant taste. Moreover, as these containers are processed duringmanufacture, after the drawing and ironing step, conditions on theprocessing line can lead to corrosion of the exposed steel surface,leaving unsightly rust marks and blemishes rendering the containersunacceptable for use. Any corrosion and blemishes formed on the surfacewill adversely affect the adhesion of any subsequent sanitary lacquercoat which is applied thereafter.

It has become customary to provide containers with a conversion coatingimmediately after the drawing and ironing step, prior to applying thesanitary lacquer, so as to prevent corrosion and improve the adhesion ofthe sanitary lacquer. Chromate passivation treatments have been employedby the art to provide the conversion coating, and are ordinarilyeffected by cleaning the surface with a conventional alkaline cleaner,thereafter applying to the surface a solution having chromic acid orsalts thereof. The deposition of chromates has proven to be detrimental,especially in the foodstuffs industry due to the high toxicityassociated with hexavalent chromium. The deposition of hexavalentchromium to the container has also become unsuitable because of thedifficulties inherent in removing the residues of the chromate solutionfrom the surface prior to deposition of a siccative finish or sanitarylacquer. In addition, an ancillary problem associated with the use ofchromate treating solutions is that the bath effluent cannot bediscarded without treatment to reduce or eliminate the chromatetoxicity.

It is an object of the present invention to provide a composition forthe treatment of drawn and ironed tinplated ferrous metal containerswhich will enhance the corrosion resistance of container surfaces andimprove the adhesion properties of a later applied siccative finish,such as sanitary lacquer.

An additional object of this invention is to provide a high-speedcoating process for tin-plated ferrous metals.

A further object of this invention is to provide a one package coatingcomposition for use on tin-plated ferrous metal surfaces, therebyfacilitating easy handling and offering high stability.

A concomitant object of this invention is to provide a coating processfor tin-plated ferrous metal containers to deposit a conversion coatingof the type known in the art as an iron phosphate coating.

DETAILED DESCRIPTION OF THE INVENTION It has been discovered that anaqueous solution comprising a primary and/or secondary phosphate, ahydroxylamine salt, and a fluoride salt when applied to drawn and ironedtin-plated ferrous metal surfaces will provide an excellent corrosionresistant coating which also enhances the adhesion of a later appliedsiccative finish.

It should be understood that the term ferrous metal used herein includesa wide variety of steels, iron, and iron alloys.

The solutions to be used in the present invention preferably areprepared by employing a one package aqueous concentrate consisting ofthe primary or secondary phosphate, hydroxylamine salt, and an alkali orammonium fluoride or bifluoride. This one package aqueous concentratecan be added directly to water in order to prepare the aqueous solutionfor use. The one package aqueous concentrate offers the advantage ofhigh stability for prolonged periods of time.

The aqueous concentrates to be employed to make up the aqueous coatingsolution may contain a primary or secondary phosphate, a fluoride orbifluoride salt, and a hydroxylamine salt in varying concentrations,since the concentrate is added to appropriate amounts of water toprepare a working solution having constituent concentrations within theoperative ranges set forth hereinbelow.

The primary and secondary phosphates suitable for use herein are amonobasic or dibasic monophosphate or a mixture of both the monobasicand dibasic monophosphate. Condensed phosphates should be avoided, sincethey have a tendency to hydrolyze in acidic solutions thereby adverselyaffecting the operating parameters of the working bath. Themonophosphate can be incorporated in the aqueous concentrate in anyconvenient form which is soluble. The monophosphate can thus beintroduced, for instance, in the form of either sodium, potassium, orammonium phosphate. It is preferred that monosodium phosphate beemployed, as it is readily soluble and contributes to optimum stabilityof the coating bath. The alkali or ammonium phosphate can be present inthe coating solution in an amount from about 1.0 to about 5.0 percent byweight. It is pre ferred that the phosphate salt be present in amountsfrom 1.3 to 1.6 percent by weight.

The hydroxylamine salt to be utilized to prepare the coating solutioncan be in the form of any soluble salt, such as hydroxylammonium acidsulphate, hydroxylammonium nitrate, and hydroxylamine hydrochloride. Thehydroxylammonium salt should be present in the working coating solutionin an amount from about 0.07 to about 0.5 percent by weight. Preferablyfrom about 0.14 to about 0.17 percent by weight will be used.

The source for fluoride in the aqueous concentrate can be an alkalibifluoride, ammonium fluoride or bifluoride, such as sodium fluoride,sodium befluoride, ammonium fluoride, ammonium bifluoride, potassiumfluoride, and potassium bifluoride. A complex fluoride can be employedin the present invention, providing that the quantity of complexfluoride present will be that which will yield or liberate free fluoridein amounts equivalent to the prescribed amount necessary when a fluorideor bifluoride salt is used. For example, the source of fluoride can befluosilicic or fluotitanic acid.

A drawback in employing a complex fluoride is that as various saltsbuild up in an operating solution during continuous use, hydrolysis willordinarily not be sufficient to supply the necessary amounts of freefluoride. Accordingly, the preferred process of this invention utilizesthe simple alkali fluoride or bifluoride salt. The fluoride orbifluoride salt should be present in the aqueous coating solution in anamount from about 0.03 to about 0.5 percent by weight. For optimumresults, it is preferred that the fluoride salt be present in an amountfrom 0.07 to about 0.1 percent by weight.

The free fluoride in the coating solution aids in the attack and removalof residual oxides on the metal substrate which have formed thereonduring or after the forming or drawing and ironing operation, and whichwould ordinarily impede deposition of a uniform coating. The removal ofany oxides on the substrate facilitates more efficient and uniformcoating information. A surprising aspect of this invention is that thecoating process can be effected when the amount of free fluoride in thecoating solution, having constituent concentrations as specified herein,is as low as 0.15 gms/liter.

Typical examples of concentrated aqueous formulae, suitable for dilutionwith additional water to make a coating solution of optimum qualityhaving constituent concentrations and operating parameters within theranges set forth herein, are as follows:

These makeup concentrates include all constituents which will beemployed in the coating solution, thereby allowing for easy preparationof the coating solution by simple addition of the concentrate to water.Of course, it is desirable in certain instances to add each of theconstituents to water separately in order to prepare the aqueous coatingsolution. In addition, as is specified herein, appropriate additions ofacid or base may be necessary to adjust the operating pH of the coatingsolution during use.

In the preferred embodiment of this invention, an operating aqueoussolution is employed comprising monosodium and disodium monophosphate,hydroxylammonium acid sulphate, and ammonium bifluoride. Optimum resultswill be obtained when the concentration of constituents in the solution,at the beginning of the operation, as well as upon replenishment, ismaintained such that the disodium and monosodium monophosphate togetherare present in an amount from about 1.3

to about 1.6 percent by weight. the hydroxylammonium sulphate is presentin an amount from about 0.14 to about 0.17 percent by weight, and theammonium bifluoride is present in the amount from about 0.07 to about0.1 percent by weight. When a coating solution is operated andmaintained within these prescribed limits, it has been found thatexcellent corrosion resistance of the tin-plated ferrous metal surfacewill result. A most surprising result is that the surface will maintainits original appearance, that is a highly polished look. The coatingproduced will also improve the adhereance of a subsequently appliedsanitary lacquer, such as an acrylic base coat.

The coating process is normally employed after cleaning of the metalsurface has been accomplished. The cleaning steps are ordinarily carriedout after the tin-plated metal has been drawn and ironed to form thecontainer. The cleaning step can be carried out by conventional methodswhich form no part of the present invention. A conventional alkalinecleaner can be employed followed by a water rinse. Should the surface beheavily soiled, a detergent cleaner additive may be employed in thecleaning step.

1n the preferred embodiment of the present invention, the metal surfaceshould be cleaned employing techniques that obtain a completelywater-break-free surface in a short period of time. An alkaline cleanerhaving the following formula can be employed:

7: by Weight Borax Pentahydrate 60 0 During the coating operation,depletion of the constituents in the coating bath will occur at aboutthe same rate. These losses must be replaced to maintain the bath withinits optimum operating limits. The coating bath is maintained within itsprescribed limits with suitable additions of a replenishing concentratehaving the constituents therein in the same proportions in which theseconstituents exist in the operating bath.

In the process of the present invention, the tin-plated ferrous metalsubstrate is brought into contact with the aqueous coating solutionunder suitable conditions of pH, temperature, and contact time.

The time of treatment of the metal surface with the coating solutionneed only be long enough to insure complete wetting of the surface andcan be as long as 5 minutes. Preferably, contact time between metalsubstrate and coating solution should be from about one second to aboutone minute. One of the distinct advantages of the present invention isthat a suitable protective coating can be formed on tin-plated ferrousmetal surfaces utilizing a treating time of as little as one second.

The coating process can be effected by employing any of the contactingtechniques known to the art. Contact can be effected by spraying orimmersing the workpiece to produce the desired surface coating.Preferably, the coating solution will be applied to the metal surface byconventional spray methods. When spray techniques are employed, lowpressure application of the coating solution has proven effective indepositing satisfactory coatings on the workpiece. Nozzle pressures aslow as 10 20 psi have been employed with excellent results.

The pH of the treating solution should be maintained at a level withinthe range of from about 4.5 to about 5.5. Preferably, the solution isoperated at a pH of from about 5.0 to about 5.4. It should be understoodthat a suitable acid, such as phosphoric acid, or base, such as sodiumhydroxide, can be added to the coating solution to adjust the pH of thebath within the operating limits prescribed herein. The use of thecoating solution having a pH above 5.5 is to be avoided, as little or nocoating will be produced. It should be noted that operation of thecoating solution below a pH of about 4.5 will result in too vigorousattack of the tin-plated ferrous metal surface.

The coating process can be operated at a temperature from about 75F. toabout 150F. It is preferred to operate the coating bath at a temperaturefrom about 120F. to about 140F. Generally, a slight change in thetemperature will not necessitate substantial alteration of treatingtime, concentration parameters, or pH adjustment.

Following the application of the coating solution, the coated surfacecan be rinsed with water or a conventional final rinse and then dried.The water rinse is necessary to remove any remaining residues which mayhave remained after thecoating step.

For optimum results, after the coating solution has been applied to thesurface, a water rinse followed by a deionized water rinse can beutilized.

The method of the present invention can be effected by treating thetin-plated ferrous metal surface after the drawing and ironing step.Generally, the coating solution will be applied directly after theforming operation or a short period of time thereafter.

Once the coating and rinse steps are accomplished, the surface can bedried by conventional means and a siccative finish, such as a sanitarylacquer, can be applied to the surface with good adhesion results. Afterthe drying step and application of the siccative finish, the workpieceis ready for use and will not suffer corrosive attack from any liquid orfoodstuff placed in the container.

A particular advantage of the present invention is that after thecoating and water rinse stages have been accomplished, the workpiecewill not suffer corrosive attack, even when subjected to prolongedexposure to air due to processing line stoppage prior to application ofthe siccative finish.

The examples presented below are illustrative of this invention and arenot considered as limiting for other materials and operating conditionsfalling within the scope of this invention that might be substituted.

EXAMPLE 1 Tin-plated drawn and ironed steel containers were employed inthis procedure. The containers were cleaned by wiping with a solventcleaner consisting of butyl cellosolve and neutral spirits so that anyoil layer remaining from the drawing operation was removed. Thecontainers were then cleaned with an alkaline cleaner for about oneminute at about 175F. so that the metal surfaces were water-break-free.The containers were then rinsed with water.

An aqueous concentrate of Formula 4 was prepared by adding a 50 percentsolution of caustic soda to a 75 percent solution of phosphoric acid,thereby neutralizing the acid and forming the mixture of disodium andsodium monophosphate in situ. The hydroxylammonium acid sulphate wasthen added followed by additions of ammonium bifluoride and water tomake 1 liter ane complete dissolution of the constituents was achieved.

The use of phosphoric acid and sodium hydroxide in an aqueous medium,results effectively in the formation of monosodium and disodiummonophosphate and has the same effect as if the alkali monophosphatesalone were used. The choice of ingredients in this case is made forpractical economical reasons.

An aqueous coating solution was prepared by adding the aqueousconcentrate of Formula 4 to water at the rate of ml. of concentrate foreach 4 liters of water. The pH of the resulting solution was adjusted to5.0 by addition of phosphoric acid. The test containers were immersed inthe coating solution, for a period of one minute at a temperature ofabout F.

After treatment, the containers were allowed to stand for 24 hours andwere observed to have retained a bright, rust-free appearance.

Control containers were employed and were cleaned and rinsed and givenno further treatment. The control exhibited badly rusted surfaces, bothinside and out, after a period of 24 hours.

EXAMPLE 2 Tin-plated steel containers were cleaned as in Example 1,first with a solvent cleaner, then with an alkaline cleaner. The testcontainers were then sprayed with an aqueous solution having thefollowing constituents:

7: by Weight Monosodium Monophosphate 0.53 Disodium Monophosphatc 0.47Hydroxylammonium Acid Sulphate 0.12 Ammonium Bifluoridc 0.06 Water 98.82

The pH of the above solution was measured at 5.3. The solution wasapplied to the containers at a pressure of about 20 psi and at atemperature of about 125F. A spray time of one minute was utilized.

The containers were then rinsed with water, followed by a deionizedwater rinse, and dried at ambient temperature. The containers hadretained a rust-free appearance after treatment. I

One set of control tin-plated steel containers was also employed in thisprocedure. The controls were cleaned with a solvent cleaner, then withan alkaline cleaner, and were given no further treatment.

The test containers were permitted to stand for a period of 72 hours andwere then subjected to humidity testing and rust testing. In thehumidity test, 10 ml. of tap water was placed in each test container,which was then covered with a watch glass and allowed to stand for onehour in an oven at 100F. The containers were then examined and evaluatedfor any rusting. They showed little rusting and still retained theiroriginal untamished, bright appearance. The control containers showedbad rusting.

In the rust test, a group of test containers were evaluated for exposediron pores. Strips of filter paper were immersed in a solution of KFe(CN) and then placed in contact with the interior of each containerfor one minute. The appearance of blue stains indicated the presence ofexposed iron pores which are susceptible to rusting. All test containersexhibited no staining. The

controls portrayed staining over much of their surfaces.

ent in an amount from about 0.07 to about 0.5 percent by weight, saidsolution having a pH from about 4.5 to about 5.5; thereafter rinsing thesurface with water; and thereafter applying a siccative finish to thesurface.

2. The method of claim 1 wherein the monophosphate is present in anamount from about 1.3 to about 1.6 percent by weight, the fluoride saltis present in an amount from about 0.7 to about 0.1 percent by weight,and the hydroxylamine salt is present in an amount from about 0.14 toabout 0.17 percent by weight.

1. A METHOD FOR COATING A TIN-PLATED FERROUS METAL SURFACE COMPRISINGTHE STEP OF: CLEANING THE METAL SURFACE; APPLYING OT THE SURFACE ANAQUEOUS SOLUTION CONSISTING ESSENTIALLY OF A MONOPHOSPHATE SELECTED FROMTHE GROUP CONSISTING OF MONOBASIC AND DIBASIC MONOPHOSPHATE AND MIXTURESTHEREOF PRESENT IN AN AMOUNT FROM ABOUT 1.0 TO ABOUT 5.0 PERCENT BYWEIGHT, A FLURIDE SALT PERCENT BY WEIGHT, AND A HYDROXYLAMINE SAL TOABOUT 0.5 PERCENT BY WEIGHT, AND A HYDROXYLAMINE SALT PRESENT IN ANAMOUNT FROM ABOUT 0.07 TO ABOUT 0.5 PERCENT BY WEIGHT, AND SOLUTIONHAVING A PH FROM ABOUT 4.5 TO ABOUT 5.5; THEREAFTER RINSING THE SURFACEWITH WATER; AND THEREAFTER APPLYING A SICCATIVE FINISH TO THE SURFACE.2. The method of claim 1 wherein the monophosphate is present in anamount from about 1.3 to about 1.6 percent by weight, the fluoride saltis present in an amount from about 0.7 to about 0.1 percent by weight,and the hydroxylamine salt is present in an amount from about 0.14 toabout 0.17 percent by weight.